Foundry media is used in various casting processes in the metal casting industry. The function of the foundry media is as a backing, core, or molding media. The backing media is unbonded, supporting both internal and external areas of a preformed pattern; the core media is resin-bonded together and produces the internal open cavity of a casting; and the molding media is resin or clay-bonded and produces the external body of a casting. In such casting processes, molten metal is poured into a molded area in the presence of the foundry media to produce a casting of designed shape, size, and dimensions. As the molten metal is poured into the mold, the foundry media is heated and expands. When the metal and the mold cool to room temperature, the metal and the mold will contract. The expansion and contraction can result in defects in the resulting cast metal part.
The degree and rate of expansion that occurs can vary by the type of foundry media used. The coefficient of thermal expansion represents the amount a material will expand or contract upon heating or cooling. Foundry media with smaller coefficients of thermal expansion and a more linear rate of expansion will have less expansion (and more uniform) contraction during use as a molten metal mold, core, or backing material. This results in tighter dimensional tolerances and fewer defects in the final metal part.
Silica sand, the most common media used for metal casting applications, has a coefficient of thermal expansion of greater than ten (10−6 inch per inch per ° C.). Silica sand also goes through multiple phase changes (e.g., alpha→beta→trydamite) when heated. These expansion properties can result in a high presence of expansion-related defects and additive costs to buffer or avoid these defects. Other known foundry media include a synthetic ceramic media commercially available from CARBO Ceramics Inc. under the tradename ACCUCAST®, which has a coefficient of thermal expansion of around 6.5 (10−6 inch per inch per ° C.). Synthetic ceramic media also has a low linear expansion rate. These properties avoid expansion-related defects and various costly additives while enhancing dimensional precision and casting capability.
High thermal expansion properties can limit the ability to produce castings with thin walls or very complex parts that require high levels of dimensional precision. Foundry media with high thermal expansion properties may require additives to buffer the foundry media expansion or high machining and cleaning costs to correct for poor cast properties. Foundry media having lower thermal expansion properties can benefit the foundry industry through: (1) reduced casting defects; (2) reduced pre-engineering costs; (3) enhanced thin-wall capabilities; (4) enhanced capabilities for producing castings of high complexity; (5) reduced use of high-cost expansion buffer additives; (6) reduced use of costly and time consuming washes and their associated equipment and workers; (7) reduced cleaning and machining time and cost associated with cleaning or correcting the final cast products; and (8) reduced scrapped casting.
While synthetic ceramic media that are commercially available have lower thermal expansion properties, they can have some disadvantages when making chemically-bonded cores and molds using a resin. Certain resin materials are oftentimes used as coatings on the foundry media grains to bond the foundry media grains together with sufficient strength for retaining the core and mold shape after the core and mold package have been formed therefrom. The strength of the formed cores and molds plays a role in retaining shape and dimensional tolerance while the molten metal fills all cored and mold cavities. The amount of resin required to coat the foundry media such that the foundry media has sufficient strength to retain its shape once molded has an influence on the overall cost of the molding operation. While a higher level of resin can be used to increase the strength of the mold, this will not only increase costs, but can result in degradation of the metal casting. During the metal casting, the resin in the core and mold package gets heated by the molten metal to such a point that the polymer becomes a gas and “burns out”. In addition, the extra gas created from the extra resin can exceed the permeability between the foundry media grains such that the gas cannot “escape” and will remain at the metal/mold surface, resulting in a defect in the cooled cast metal surface or will become entrapped in the metal, weakening the metal, creating a potential failure point in the final metal casting.
What is needed, therefore, is a synthetic foundry media that has a low coefficient of thermal expansion and minimizes the amount of resin applied thereto.